As a supercharger for supercharging intake air by using exhaust power, a turbocharger is generally known. In recent years, an electrically-assisted turbocharger is developed in which an electric motor or the like is attached to the rotary shaft of the turbocharger and which assists exhaust power in accordance with an operating state of an internal combustion engine.
A controller disclosed in JP-11-117933A controls current passed to an electric motor on the basis of an accelerator stroke amount and accelerator stroke speed, thereby advancing supercharging operation of a turbocharger. A controller disclosed in JP-11-280510A controls the speed of opening the throttle and current passed to an electric motor in accordance with a change amount of an accelerator position and, meanwhile, corrects an auxiliary power amount in accordance with engine speed and a load value. In such a manner, supercharging according to the accelerator operation of the driver is realized to improve drivability. A controller disclosed in JP-2003-239754A determines power to be supplied to an electric motor on the basis of a power determination reference such as a map in which the relation between target boost pressure and power supplied to the electric motor is specified in advance and, further, corrects the power determination reference on the basis of actual boost pressure fluctuations with respect to the power supplied to the electric motor. By the operations, the boost pressure control which is always optimum can be executed.
In the conventional methods, however, open-loop control is executed, and it is difficult to control the assist amount properly. In this case, when the assist amount increases unnecessarily, there is the possibility that fuel consumption deteriorates. There is also the possibility that supercharging performance (acceleration performance) intended by the driver cannot be obtained at the time of acceleration or the like, and drivability deteriorates.
JP-2002-21573A discloses a technique of providing an auxiliary compressor as an auxiliary supercharging device on the upstream or downstream side of a turbocharger in an intake path. The auxiliary compressor is operated by, for example, an electric motor.
However, the above publication does not disclose a control method of how to control the auxiliary compressor. It is therefore difficult to operate the auxiliary compressor with a proper control amount. For example, the configuration in which the auxiliary compressor is operated by an electric motor or the like has the possibility that when the auxiliary compressor is operated unnecessarily, a power generation amount of an alternator or the like increases and, as a result, fuel consumption deteriorates. When an operation amount of the auxiliary compressor is insufficient, supercharging performance (acceleration performance) intended by the driver cannot be obtained at the time of acceleration or the like, so that drivability may deteriorate.